This invention relates to a double operate check valve inserted in a hydraulic circuit for locking and unlocking a hydraulic cylinder used in a suspension system for a vehicle such as automotive travelling crane.
When an automotive travelling crane moves as it hangs a load, its suspensions are subjected to expansion and contraction which are liable to result in unstable travelling attitude. In order to avoid such trouble, there has been provided a hydraulic circuit which can lock and unlock the suspensions as disclosed in the Japanese Utility Model Publication No. S63-21115.
As shown in FIG. 1, the disclosed hydraulic circuit includes hydraulic cylinders 10 each disposed between a cylinder frame 40 and a spring which suspends a wheel 42 on the frame 40, as shown in FIG. 4. The suspensions are locked by feeding compressed oil to the cylinders 10 and unlocked by connecting the cylinders 10 to an oil tank. The circuit also includes double operate check valves 11 each connected to input and output ports 10A and 10B of each hydraulic cylinder 10 as shown in detail in FIG. 2.
As shown in FIG. 2, the double operate check valve 11 includes a valve body 1 having both ends closed with plugs 1a and 1a, a pair of parallel fluid paths 2 and 2 for feeding compressed oil to the cylinder 10 and discharging it therefrom and a pair of valve mechanisms 3 and 3 disposed in the way of the fluid paths 2 and 2. Each valve mechanism 3 comprises a valve seat 4, a poppet valve 5 and a spring 6 for urging the poppet valve 5 against the valve seat 4. The poppet valve 5 comprises a check valve 7 urgeable against the valve seat 4 and a small valve 8 sliding within the check valve 7, and the small valve 8 is urged toward the check valve 7 by the spring 6.
A cylindrical insert hole 9 connecting both fluid paths 2 and 2 is formed in the valve body 1 and a pair of pilot pistons 13 and 13 are inserted therein so that they can slide in the insert hole 9 and touch the poppet valves 5 and 5, respectively. A pilot chamber 14 is provided between both pilot pistons 13 and 13 and adapted to be fed with compressed oil through a pilot piping 21 from a compressed pilot oil source 19 by switching a switch valve 20 to a switch position 20B.
When a switch valve 17 is in its neutral position 17A and the switch valve 20 is in its mormal position 20A, as shown in FIG. 2, the pilot chamber 14 is connected through the pilot piping 21 to the oil tank, so that each check valve 7 is pushed by the spring 6 through the small valve 8 against the valve seat 4 to close it. Accordingly, hydraulic chambers 10C and 10D of each cylinder 10 are closed and the corresponding suspension is locked.
If the switch valve 17 is switched to its switch position 17B, a pump 18 is connected to a piping 16 and a piping 15 is connected to the oil tank. Therefore, the compressed oil from the pump 18 flows in one of the fluid paths 2 of the double operate check valve 11 and opens corresponding one of the valve mechanisms 3, so that it is fed to the hydraulic chamber 10D of the cylinder 10. The compressed oil flowing into the abovementioned fluid path 2 acts to the pilot pistons 13 and 13 and opens the other valve mechanism 3 to discharge the oil from the hydraulic chamber 10C of the cylinder 10 to the oil tank, thereby contracting the cylinder 10.
When the switch valve 17 is switched to its switch position 17C, the pump 18 is connected to the piping 15 and the piping 16 is connected to the oil tank. Therefore, the compressed oil from the pump 18 flows into the other fluid path 2 on the doubld operate check valve 11 and opens the corresponding valve mechanism 3, so that it is fed to the hydraulic chamber 10C of the cylinder 10. The compressed oil flowing in the abovementioned fluid path 2 acts to the pilot pistons 13 and opens the other valve mechanism 3 to discharge the oil from the hydraulic chamber 10D to the oil tank, thereby expanding the cylinder 10.
If the switch valve 17 is put in its neutral position 17A and the switch valve 20 is switched to its switch position 20B, the compressed pilot oil source 19 is connected to the pilot piping 21 and the compressed oil from the source 19 flows in the pilot chamber 14 of the double operate check valve 11 and acts to both pilot pistons 13 and 13 to open both valve mechanisms 3 and 3. Accordingly, both hydraulic chambers 10C and 10D of the cylinder 10 are connected through the switch valve 17 to the oil tank, thereby unlocking the cylinder 10 for free operation.
According to the prior art double operate check valve as described above, the switch valve 17 is put in its neutral position 17A to connect the pipings 15 and 16 to the oil tank and the switch valve 20 is switched to its switch position 20B to feed compressed pilot oil to the pilot chamber 14, thereby releasing the valve mechanisms 3 to unlock the suspension to allow it to absorb the vibration due to road surface undulation and to obtain comfortable drive.
However, the hydraulic chambers 10C and 10D of the cylinder 10 are adapted to connect to the oil tank through the double operate check valve 11 and the pipings 15 and 16 and these pipings are extended over the whole length of the vehicle. Therefore, the oil fed to and discharged from the hydraulic chambers 10C and 10D is subjected to substantial pipe resistance and the action of the cylinder 10 can not follow enough the vibration due to road surface undulation, thereby resulting in uncomfortable drive.
Therefore, an object of this invention is to reduce the amount of the oil fed to and discharged from the hydraulic cylinders of the vehicle suspensions so that the suspensions can sufficiently follow the road surface undulation, thereby enabling comfortable drive.